In recent years there has been an increasing interest in generating ethanol and fine chemicals from lignocellulosic feedstocks. These feedstocks are of particular interest as they are inexpensive and are often burned or landfilled. Accordingly, there is an enormous untapped potential for their use as a source of fermentable sugar to produce ethanol or other byproducts. The fermentable sugar is produced from the polysaccharide components of the feedstock, namely cellulose which makes up 30% to 50% of most of the key feedstocks, and hemicellulose which is present at 15% to 30% in most feedstocks. The remaining components of lignocellulosic feedstock include lignin, which is typically present at 15-30%, ash, protein and starch.
In order to produce sugar from lignocellulosic feedstocks, it is first necessary to break the polysaccharides down into their composite sugar molecules and this is typically accomplished by physical and/or chemical pretreatment, followed by hydrolysis of the cellulose. An example of a chemical pretreatment is acid pretreatment (see U.S. Pat. No. 4,461,648), which hydrolyzes most of the hemicellulose to xylose, galactose, mannose and arabinose, but results in little conversion of the cellulose to glucose. The cellulose may be hydrolyzed to glucose by cellulase enzymes or by further chemical treatment with acid.
It is also known to hydrolyze the lignocellulosic feedstock in a single step with acid that utilizes harsher conditions to effect hydrolysis of both the hemicellulose and cellulose components of the feedstock.
Glucose can be fermented to fuels including, but not limited to, ethanol or butanol or other chemicals, examples of which include sugar alcohols and organic acids. The pentose sugars, xylose and arabinose, can be fermented to ethanol by recombinant yeast (see U.S. Pat. No. 5,789,210 (Ho et al.), U.S. Pat. No. 5,126,266 (Jeffries et al.), WO 2008/130603 (Abbas et al.) and WO 03/095627 (Boles and Becker)) or by bacteria. Moreover, the production of xylitol from xylose has received much attention because of its value as a substitute sugar sweetener.
The development of a continuous process to produce ethanol or other fermentation products that can be operated and maintained economically has been the goal of various researchers in the field. Reductions in throughput compromise productivity, which can translate into significant cost. To maintain a continuous, high throughput process, the flow rate throughout the process should be consistent. If the flow rate through one stage of the process is reduced, subsequent stages are similarly affected, which ultimately reduces the throughput of the entire system. Moreover, interruptions to the process can also significantly reduce the efficiency of the process.
It was unexpectedly discovered that a solid scale deposit (referred to herein as “scale”, “scale deposit” or “pretreatment scale”) can accumulate within a pretreatment reactor. Once discovered, it was found that this was inhibiting the flow of the feedstock slurry through the reactor and thus through downstream stages of the process. The scale accumulation necessitated frequent halting of the process for cleaning operations and this proved to be a time-consuming and costly endeavor as the system had to be disassembled and then subjected to a high pressure water wash. The identity of the scale was unknown until investigative work revealed that it was composed of lignin.
WO 2006/128304 discloses that scale containing inorganic salts can deposit on the process equipment downstream of a pretreatment reactor. Deposition of scale occurs after the addition of alkali to adjust the pH of the acid-pretreated slurry exiting the reactor to 4-6 prior to enzymatic hydrolysis. This scale deposit contains calcium sulfate and calcium bisulfate resulting from the sulfuric acid added during pretreatment and calcium that is present in the feedstock.
Lignin is an organic compound that confers water resistance and stiffness to the fiber, as well as protection against microbial attack. Lignin differs from cellulose and hemicellulose in that it is not composed of sugar units, but rather a complex three-dimensional matrix of phenolic-propane units. Although lignin does not yield any fermentable sugars, it can be burned in the plant to generate electricity for the conversion process, thereby avoiding the use of fossil fuels. Lignin that is burned in the plant is obtained from downstream stages of the process, typically after cellulose hydrolysis by cellulase enzymes. Insolubles that remain after enzymatic hydrolysis contain insoluble lignin and can be separated from the sugars by filtration and then burned in a boiler to generate steam.
It is known to remove lignin from the lignocellulosic feedstock itself at the beginning of cellulosic conversion processes and numerous reagents have been proposed for such purpose. Examples include treatment with ethanol and water, followed by hemicellulose hydrolysis at high temperatures (WO2007/129921); delignification by pretreatment conducted with lime under oxidative conditions, followed by enzymatic hydrolysis of cellulose (US 2008/0121359), dissolution of lignin and release of monosaccharides, polysaccharides and oligosaccharides by cooking lignocellulosic feedstock with organic solvents (e.g. ethanol), followed by saccharification with cellulase enzymes and fermentation (U.S. Pat. No. 7,465,791); and aspen chip delignification with monoethanolamine (Shah et al., 1991, Applied Biochemistry and Biotechnology 28/29:99-109). Enzymatic degradation of lignin has also been investigated (Eriksson, 1993, Journal of Biotechnology 30:149-158). However, none of the foregoing references disclose the production of lignin scale, or any measures that can be taken for its prevention or removal.
EP 1,026,312 discloses a method for purposefully precipitating dissolved lignin onto pulp fibers to improve pulp yield of unbleached pulp. Such precipitation is accomplished by adding an acid into a dilute lignin stream during the washing stage of brown stock pulp. The pH of the dilute lignin stream is reduced to a level sufficient to cause the precipitation of lignin onto the pulp fiber and to increase pulp yield. However, in the process the acid concentration is low enough that caking or blockages in piping or on the pulp washer are prevented.
U.S. Pat. No. 3,546,200 discloses a method for precipitating lignin from black liquor, which is the liquid material remaining from pulpwood cooking in the soda or sulfate papermaking process. The process involves acidifying the lignin-containing black liquor to a pH of less than 4.1 and treating the mixture before a step of filtering with a hydrocarbon that enhances the precipitation.